Telephone services are used in connection with broadband data such as cable modems, constant bit rate (CBR) modems in hybrid fiber-coaxial (HFC) system, and digital subscriber line (DSL) modems in twisted-pair copper systems to provide data communications. Generally, these systems provide battery, dial tone, supervisory signals, ringing and other standard Plain Old Telephone Service (POTS) signals over a two wire interface through connectors (e.g., RJ11) to telephones, facsimile machines and dialup modems.
A HFC or DSL system provides a second line service that coexists in a subscriber's residence with a primary line service, for example, an Incumbent Local Exchange Carrier (ILEC) service. The subscriber could inadvertently connect a telephone cord from the second line device (the HFC or DSL system) to a jack which is connected to the primary line service (the ILEC service). This possibly results in permanent damage to the circuitry of the ILEC central office or the HFC/DSL system. It could also interfere with proper functioning of the primary line service and could effectively disable E911 service from the residence.
One exemplary method for detecting presence of service is testing for foreign voltage on a telephone line, which detects cases of the telephone line being shorted to an AC power line or a 48V battery in a central office. However, the test can also detect the shorts between two adjacent telephone pairs in a wire bundle. For example, a voltage sensor in a second line device detects presence of DC voltage on a telephone line before a Subscriber Line Interface Circuit (SLIC) of the second line device is enabled to drive voltage onto the telephone line. If a voltage on the telephone line is above a predetermined threshold value, the SLIC is disabled. This method only works when the second line device is first enabled. However, since the connection to a primary line device could be made at any time and both the primary and second services would provide about 48V to the telephone line, the second line device would likely not be able to detect that a connection to another service has been made.
Another method for detecting presence of service is for the second line device to reverse voltage polarity on a telephone line while the primary line device provides normal polarity voltage to the line (in this case, assume no off-hook devices are connected to the line). If a primary line device is driving the telephone line, this will cause a current to flow, which will be detected as an off-hook condition. However, this method causes temporary disruption and unavailability of the telephone line and could cause temporary over-current condition in a SLIC of a device, especially if the method is performed just as a central office sends a power ring. Some line-attached devices sense line polarity reversal as a precursor to other signals, so it could cause them to malfunction. Further, the primary line device may sense a fault condition and limit the current to a level less than the hook detect level of the SLIC, which may make the test unreliable.
Yet another method for detecting presence of service is to disable the SLIC (again assume that no off-hook devices are connected). The SLIC is disabled when it is placed in a high impedance state to effectively disconnect it from the telephone line. On a telephone line with several connected devices, none of which is off-hook, the time constant due to capacitance in the telephone line and in attached devices, combined with no return path for DC currents, could be several seconds. Thus, the detection method would not be valid until a long period of time has elapsed. Since the telephone line would be out of service during that interval, this method may not be acceptable. One exemplary method for avoiding this problem is disclosed in “Loop Testing per TA-909 with the L9310/L9311 SLIC Circuit”, Lucent Technologies Microelectronics Group, Application Note AP01-018ALC, January 2001. This note describes rapidly reversing voltage polarity of a telephone line just before measuring the voltage to quickly clear any charge on the line. In the note, the polarity is reversed every 150 usec during an interval of 100 msec.
A further method for detecting presence of services is detecting temperature fault in a SLIC. The temperature fault could indicate that an external device is driving excess current into the SLIC. However, unknown causes of an over-temperature fault may result in damage to the SLIC or central office, and can disturb a telephone line for a long period of time.
Thus, a need exists for a method of detecting presence of service without damage to a SLIC or central office equipment.